Our principal objective is to determine the three-dimensional structures of immunoglobulins from patients with multiple myeloma and/or amyloidosis. We are currently emphasizing the serum IgG1 protein and the urinary Bence Jones dimer from the patient Mcg. A low resolution (6.5-Angstrom) structure of the Mcg IgG1 protein is currently being extended to higher resolution with the use of the "CORELS" program for rigid-body refinement. Joel Sussman at NIH kindly updated this program for our use. The Mcg Bence Jones dimer crystallizes in a trigonal form in ammonium sulfate and in an orthorhombic form in water. We previously solved the structure of the trigonal form to 2.3-Angstrom resolution and built an atomic model. An electron-density map for the orthorhombic form has now been interpreted at 2.7-Angstrom resolution with phase extension and refinement techniques. It is already clear that there are marked differences in the crystal structures of the Mcg Bence Jones dimer in the two solvent systems. These results emphasize that one amino acid sequence in an immunoglobulin gene product can give rise to multiple three-dimensional conformations. The structure of a hybrid of the Mcg light chain with the Bence-Jones protein from the patient Weir has been solved to 6.5-Angstrom resolution. Despite 37 differences in the amino acid sequences of the Mcg and Weir light chains, the structures of the Mcg native dimer and the Mcg x Weir hybrid are very similar. Crystals of other Bence-Jones proteins, hybrids of light chains, and antigen-binding fragments from monoclonal antibodies have been obtained and are currently being subjected to X-ray analysis. (AB)